This invention relates broadly to gas-liquid contacting apparatus, in which mass transfer occurs between a gas and a liquid flowing in opposite directions and brought into contact with each other. For example, in one type of such apparatus specifically known as an air stripper, air and water are supplied to such an apparatus to remove volatile compounds from the water. Other examples of gas-liquid contacting apparatus are air scrubbers and distillation towers. All such apparatus may make use of the invention to advantage and are regarded as falling within the scope of this invention. However, the invention finds its preferred embodiment at present in the aforementioned air stripper apparatus and will be described specifically herein in connection with such type of apparatus as the preferred embodiment of the invention.
All of the aforementioned apparatus are sometimes referred to as systems. Although that term is not used specifically herein, the term apparatus is used herein to refer to systems, devices and apparatus generally and collectively.
This invention then relates specifically in one embodiment to a gas-liquid contacting apparatus of new and improved design for use in the removal of volatile organic compounds (VOC) from water and other liquids by air stripping. Water only is described specifically herein but this term is to be taken as meaning any liquid containing volatile compounds. Likewise, air only is described specifically herein but this term is to be taken as meaning any gasses capable of extracting volatiles from a liquid on contact therewith.
Current apparatus used for the decontamination of water and other liquids (herein generally described as water) by air stripping include the well known stripping towers and apparatus of the type shown in the following patents: 4,869,832; 4,954,147; 4,663,089; and 5/045,215, just to mention a few. There are improved air stripper designs as shown in co-pending patent application U.S. Ser. No. 08/043,264, filed Apr. 6, 1993 and assigned to the same assignee as is the subject application U.S. Pat. No. 5,378,246l; issued Jan. 3, 1995.
That design is of the type which includes a perforated floor or sieve tray in the treatment area of the apparatus. More specifically, the design is a gas-liquid contacting apparatus in which a plurality of vertically-spaced horizontal trays are provided with a plurality of openings through which air is permitted to pass upwardly therein through. Each tray is also provided with downcomer means at opposite sides, one for conducting water from the tray above, the other for conducting water to the tray below. Liquid water flow over each tray is directed from the one downcomer from above to the other downcomer leading below. Each next lowermost tray is preferably of substantially the same configuration as the one above it.
As already noted, the trays are provided with a plurality of openings so that air is allowed to bubble upwardly through the flowing water. The air rising through the openings thus contacts the water flowing across the tray. Water level on the tray is established and is removed from the tray by flowing over a weir (a vertical plate or fence) of predetermined height and downwardly through a downcomer to the tray below.
The water preferably flows straight across the tray as specifically described herein but may flow in a meandering path as determined by the placement of baffles thereon to direct the flow path and increase the contact between the upwardly flowing air and the water providing more opportunity for transfer of any volatile contents of the water into the air.
All of the sieve tray type air stripper apparatus to date suffer from a start-up disadvantage in that during start-up the air which is introduced at the bottom of the apparatus tends to flow upwardly through the downcomers of the apparatus since they offer a path of least resistance to the air flow rather than flowing through the apertured processing tray floors (sieve trays) as occurs during normal operation of the apparatus. Consequently, since the water to be stripped is introduced at the top of the device, the stripping action does not occur in the upper trays immediately upon start-up but is somewhat delayed. As a result of this situation the initial water flowing through the apparatus is not stripped at all or is not stripped adequately as will occur once the apparatus is in full normal operation.
Also, during initial start-up with the air following the path of least resistance, i.e. , flowing upwardly through the downcomers, there is little back pressure offered to the blower, which is often a centrifugal blower type. Thus, the blower tends to run at higher flow rate and higher amperages than necessary.
This invention provides in its most preferred form a new simplified modular air stripper design which is easy to manufacture and service. The design is readily adaptable to a variety of sizes and includes a plurality of stacked stripping trays, each of which includes an apertured floor, with opposite ends defining a flow path therebetween for water undergoing stripping. The water, containing VOC's for example, is introduced into one end of an upper tray and flows across that tray to the other end and down to a lower tray through a downcomer and so forth. Air is forced through each apertured tray floor starting in the lowermost tray and exiting out the uppermost tray. As the gas bubbles through the water it strips the VOC's from the water. Stripped water exits the apparatus from a sump chamber below the bottom tray.